Understanding Generators in Javascript - Javascript weekly

ganeshmani profile image GaneshMani Originally published at cloudnweb.dev ・5 min read

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In this weekly series, we will see what is generators in javascript and use cases of generators. Understanding Generators in Javascript - Javascript weekly

What is Generators ?

Firstly , Let's try to understand this concept with a simple analogy.

For example, Let's say that you are watching a movie on your laptop. suddenly someone at your doorbell, it is a pizza that you have ordered. you will pause the movie and get the pizza, come back and resume from the place where you left watching it. right?

Similarly ,Generators in javascript works in the same concept.

Generators are special functions in javascript which can be paused and resume from the place it left.

Moreover, we all know that once a normal javascript function gets executed, it is not possible to interrupt and do somework and comeback where it left off.

For example, Take a look at this code below

  console.log("Hello World");

function foo() {
  // NOTE: don't ever do crazy long-running loops like this
  for (var i=0; i<=700; i++) {


However, setTimeout function will only execute after the loop ends. it is not possible to interrupt the for loop and execute the function.

 this is the problem that Generators functions solves.

Implementing Generators

 To define a function as a Generator function, we need to define it with a asterik(*) to it. For example,

function* getData() {
   //yield comes here.

Every time a Generator function encounters an yield statement, it will execute it and resume the function after that.

Let's understand this concept with an example,

function* getData() {
  for(let i=0;i < 10;i++){
    console.log(yield getUser(i));


function getUser(value) {
    return {
      id : value,
      name : "John",
      age : 20 

const get = getData();


Here, we define a generator function to fetch the user data. every time the getData() function encounters the yield statement with getUser() function, it executes the function and returns the data to the main function.

we need to call the next() to iterate and get the value from the generator function. it is called as Generator Iterators.

what is Generator Iterators ?

generator iterators are special kind of design patterns. we can loop through an ordered sets step by step.

For example, consider an array with values


with an iterator, we can loop through values one at a time using the keyword next(). the purpose of doing in this way is if we have some function to run after every value returned. we can execute in this way.

function *doIt() {
    yield 1;
    yield 2;
    yield 3;
    yield 4;
    yield 5;

To step through the value of generator function doIt(). we need an iterator to be constructed. we can do that by defining,

var it = doIt();

Meanwhile, to iterate over the generator function, we need to use keyword next() to get the values.

console.log( it.next() ); // { value:1, done:false }
console.log( it.next() ); // { value:2, done:false }

we get the value and something called done which indicates whether the iterator reached the end of the function or not.

we can also use return instead of yield in the generator function. But, the problem with return is, in for..of loop return statement will be thrown away. it will not get executed

with for..of loop

function *doIt() {
    yield 1;
    yield 2;
    yield 3;
    yield 4;
    yield 5;
    return 6;

for (var v of doIt()) {
    console.log( v );
// 1 2 3 4 5

console.log( v ); // still `5`, not `6` :(

Where do we use Generator functions ?


Most importantly, redux saga is used to handle the side effects of redux. if you have worked with react/redux, you might know how difficult to maintain the action creators and reducers in redux.

Redux-saga eases the process of maintaining action-creators and reducers by grouping them called sagas.

Redux sagas explicitly uses Generators functions to do the saga effects such as

  • Calling an API and get the data from the backend
  • selecting the value from the redux state
  • take every action and do some business logic and dispatch some other action.

Promise alternative

Above all, it can also be used as a promise, async/await alternative.

Since,most of the developers used with promise, they won't use Generators that much.

But we can use Generator to solve the problem of what Promise,Async/Await solves.


OK, so that's it for the basics of generators.

Likewise, we will see more about Javascript core concepts in the upcoming Javascript weekly series. Until then, you can read my other blog posts which explain the fundamentals of web development.

Happy Coding :-)

Meanwhile ,To Read more about Javascript

You Don't Know JS

Posted on by:

ganeshmani profile



Full Stack Engineer. Currently focusing on Javascript, React, GraphQL, and Nodejs.


markdown guide

I thought generators is more like a funny toy in javascript. They have interesting concept, but almost useless in the real code. It's a surprise for me that this language feature has a real use case in redux-saga!


Generators are almost identical to enumerables in C#, the real benefit of it is reduce number of iterations. Consider following,

   // a is an array of elements

   a = a.filter((e) => some condition ....);

   if (some other condition) {
       a = a.filter((e) => some other condition ... );

   enumerate a....

In above example, a is iterated 3 times,

But with generators, you can reduce iteration to single iteration.

   ga = *(a) => for(let i=0; i<a.length; i++) {
      yield return a[i];

   filter = *(ga, filter) => { const g = ga(); while(g.next()) if (filter(g.value) yield return g.value; };

   a1 = filter(ga, (s) => s ... condition 1);
   a2 = filter(a1, (s) => ... condition 2);

   for(let item of a2) {

If you run this code, you will notice that iterating is only printed once.

You can combine various map/filter or map/reduce to only iterate an array once.


Thanks for the example. The same result might be achieved by iterating the array with the old-school for loop. The array will be iterated only once as well.

for (let i = 0; i < a.length; i++) {
  if (!condition1) continue;
  if (!condition2) continue;

I see that generators help to write code in FP-style and save iteration cycles at the same time.

This is only possible when you have your conditions when you want to iterate it, imagine series of function passing each other generators, and final evaluator does not have access to all conditions.

Conditions functions should exist somewhere. I think it's always possible to refactor the code and have access to these functions (for example, by moving the conditions functions higher in lexical scope).


Good example and explanation. :-)


Yeah true. it is one of the powerful features in javascript. sadly, it is not being used that much. but, redux saga using it in an efficient way possible.


Never needed them so far, but I do use a lot of promises.